5W30 vs 10W30: Which Engine Oil Is Right for Your Car?

When you're choosing engine oil, 5W30 vs 10W30, two common motor oil viscosity grades used in passenger vehicles. Also known as multi-grade engine oils, it's not about which one is "better"—it's about which one matches your engine’s needs, climate, and driving habits. The first number—5 or 10—tells you how well the oil flows when it’s cold. The second number—30—shows how thick it stays when the engine is hot. That’s it. No magic. No hype.

5W30, a thinner oil at startup, ideal for colder climates and modern engines with tighter tolerances. Also known as low-viscosity oil, it reaches critical engine parts faster after a cold start, reducing wear during those first crucial seconds. 10W30, a bit thicker when cold, better suited for older engines, warmer regions, or vehicles under heavy load. Also known as standard-viscosity oil, it holds up better under heat and stress, which is why many mechanics still recommend it for high-mileage cars. If you live in the UK and your car is under 10 years old, 5W30 is likely what your manual calls for. But if you’ve got a 2005 Ford Mondeo or a diesel pickup that pulls trailers, 10W30 might be the smarter pick.

Here’s what you need to know: using the wrong oil won’t instantly destroy your engine, but it can slowly hurt performance. Too thin, and you risk metal-on-metal contact under load. Too thick, and your engine works harder, burning more fuel and straining the oil pump. Modern engines are designed around specific oil grades. If your owner’s manual says 5W30, don’t swap in 10W30 just because it’s on sale. And if you’re unsure? Check the dipstick, look at the oil cap, or ask a mechanic who’s seen your car’s service history.

Don’t confuse oil viscosity with oil type. 5W30 and 10W30 can both be synthetic, semi-synthetic, or conventional. Synthetic oil, a lab-engineered lubricant that flows better, lasts longer, and resists breakdown. Also known as full synthetic motor oil, it’s not a viscosity—it’s a quality level. So you can have synthetic 5W30 or synthetic 10W30. The viscosity still has to match your engine.

And here’s a quick tip: if you’ve been using 10W30 for years and your car runs fine, don’t switch to 5W30 unless your manual says to. Engines adapt. Switching oils unnecessarily can cause leaks in old seals or upset the balance of internal clearances. But if you’re buying a new car, or your current oil is old and sludgy, sticking to the manufacturer’s spec is the cheapest insurance you’ll ever buy.

Below, you’ll find real-world guides on oil changes, engine wear, and what happens when you ignore the right oil grade. These aren’t theory pieces—they’re fixes, checks, and warnings from mechanics who’ve seen the damage wrong oil causes. Whether you’re checking your dipstick this weekend or replacing a clutch next month, the right oil is the first line of defense. Get it right, and your engine will thank you.